1. Field of the Invention
The present invention relates to an electronic endoscope apparatus for obtaining a color image signal by obtaining an image of an object to be observed by a scope unit which includes an imaging element for obtaining the image of the object to be observed by receiving light transmitted through a color filter.
2. Description of the Related Art
Conventionally, various kinds of electronic endoscope apparatuses for displaying color images based on color image signals have been proposed. In the electronic endoscope apparatuses, the color image signal is obtained by obtaining an image of an object to be observed by a scope unit which includes an imaging element on which a color filter is provided.
As an example of the electronic endoscope apparatus as described above, an electronic endoscope apparatus which obtains a color image signal including an R component, G component and B component, for example, using a single CCD (imaging element) has been proposed. The electronic endoscope apparatus is a so-called single-chip electronic endoscope apparatus.
In the single-chip electronic endoscope apparatus as described above, a color filter such as a primary color filter and a complementary color filter is used. The primary color filter includes a filter of an R component, a filter of a G component and a filter of a B component. The complementary color filter includes a filter of a Cy component, a filter of a Ye component, a filter of an Mg component and a filter of a G component.
When a color image is displayed on a monitor or the like based on a color image signal obtained by a CCD (charge coupled device) on which the color filter as described above is provided, video signal conversion processing is performed on the color image signal. However, the method for performing the video signal conversion processing is different according to the kind of the color filter. For example, if a color image signal including R, G and B components is obtained by a CCD on which a primary color filter is provided, a luminance signal Y and chrominance signals R-Y and B-Y are calculated using the color image signal including the R, G and B components without processing. The luminance signal Y and the chrominance signals R-Y and B-Y are used as video signals.
Further, generally, if a color image signal is obtained by a CCD on which, for example, a complementary color filter including Cy, Mg, Ye and G components is provided, the color image signal of these components is used to calculate a luminance signal Y and chrominance signals Cr and Cb. Further, the luminance signal Y and the chrominance signals Cr and Cb are used to calculate R, G and B signals. The calculated R, G and B signals are used to calculate a luminance signal Y and chrominance signals R-Y and B-Y. These signals are used as video signals.
Here, when a color image signal is obtained by a CCD on which a complementary color filter is provided, there is a well-known technique in readout of signals. In this technique, when the signals are read out, two adjacent pixels are mixed with each other.
However, when R, G and B components are calculated based on signal components obtained by mixing pixels as described above, the R, G and B components are not calculated based on pixel information but a mixed signal. Therefore, the R, G and B components of the adjacent pixels are mixed. Further, when the video signal is calculated as described above, the video signal is calculated within a limited range according to the hardware configuration of an endoscope apparatus. In other words, the video signal is calculated using limited numerical values. Therefore, the numerical values are rounded to an integer. Hence, a color which is reproduced from a video signal based on a color image signal obtained by a CCD on which a primary color filter is provided and a color which is reproduced from a video signal based on a color image signal obtained by a CCD on which a complementary color filter is provided are different from each other.
Therefore, even if the same color is photographed, a color which represents a different point in color space is reproduced, for example, according to a difference in the filter.
Therefore, when an image is displayed using color space that can represent each of RGB colors in 256 values, if each scope unit that has a different kind of color filter, as described above, is connected and used, the color of the displayed image is different according to the scope unit which is used.
The color image which is displayed by the electronic endoscope apparatus is provided to perform image diagnoses for observing the color of a mucous membrane, the color of a dyed mucous membrane, or the like. However, if a color representing a different point in color space is reproduced according to the kind of a color filter provided on the CCD, it is difficult to always perform accurate image diagnoses.
A technique for adjusting white balance based on the sensitivity of a CCD provided in the scope unit is disclosed in Japanese Unexamined Patent Publication No. 61 (1986)-179129. However, a difference in the kind of the color filter is not considered in the technique.